Chapter 11. ... A free radical is a species containing one or more unpaired
electrons. .... 4 the end of a longer polymer. The termination steps are the usual.
Chapter 11. Free Radical Reactions A free radical is a species containing one or more unpaired electrons. Free radicals are electron-deficient species, but they are usually uncharged, so their chemistry is very different from the chemistry of even-electron electron-deficient species such as carbocations and carbenes. The alkyl radical (·CR3) is a seven-electron, electron-deficient species. The geometry of the alkyl radical is considered to be a shallow pyramid, somewhere between sp2 and sp3 hybridization, and the energy required to invert the pyramid is very small. In practice, one can usually think of alkyl radicals as if they were sp2-hybridized.
Both alkyl radicals and carbocations are electron-deficient species, and structural features that stabilize carbocations also stabilize radicals. Alkyl radicals are stabilized by adjacent lone-pair-bearing heteroatoms and by π bonds, just as carbocations are, and the order of stability of alkyl radicals is 3° > 2° > 1°. However, there are two major differences between the energy trends in carbocations and alkyl radicals. • A C atom surrounded by seven electrons is not as electron-deficient as a C atom surrounded by six electrons, so alkyl radicals are generally not as high in energy as the corresponding carbocations. Thus, the very unstable aryl and 1° alkyl carbocations are almost never seen, whereas aryl and 1° alkyl radicals are reasonably common. • The amount of extra stabilization that adjacent lone pairs, π bonds, and σ bonds provide to radicals is not as great as that which they provide to carbocations. The reason is that the interaction of a filled AO or MO with an empty AO (as in carbocations) puts two electrons in an MO of reduced energy, whereas the interaction of a filled AO or MO with a half-filled AO (free radicals) puts two electrons in an MO of reduced energy and one electron in an MO of increased energy.
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Even though adjacent lone pairs, π bonds, and σ bonds do not stabilize radicals as much as they stabilize carbocations, the cumulative stabilizing effect of several such groups on a radical can be considerable. Benzylic radicals (those with the radical on a C atom next to a benzene ring, but not in a benzene ring) are particularly low in energy, as the radical center is stabilized by resonance with three π bonds. 11.1 Addition of HBr to Alkenes. The anti-Markovnikov addition of HBr to alkenes was probably the first free-radical addition reaction to be discovered. The discovery was inadvertent; around the turn of the twentieth century, scientists studying the regiochemistry of addition of HBr to alkenes found that the proportion of Markovnikov to anti-Markovnikov addition products varied inexplicably from run to run. Eventually it was discovered that impurities such as O2 and peroxides greatly increased the amount of anti-Markovnikov addition product (See Jones Figure 11.23). The results were later explained by a free-radical addition mechanism. Thus, 2-methylpropene will react with HBr in the presence of a peroxide such as t-BuO—O-t-Bu to regiospecifically yield the antiMarkvonikov addition product. CH3
H 3C
HBr C
H 3C
CH2
RO—OR
H 3C
C
CH2 Br
H
regiospecific, anti-Markovnikov addition
The anti-Markovnikov regiochemistry derives from the addition of Br· radical to the less substituted C atom of the alkene (steric reasons) to give the more stable, more substituted radical (electronic reasons). In a polar, electrophilic addition reaction, the regioselectivity would be
2
such that Br would add to the more substituted C atom of the alkene. Mechanism :
!
2 R—O•
:
RO—OR
or h! H
R
Br
OH
+
Br.
:
O.
:
R
: :
:
initiation steps
CH3
:
H 3C
+
Br.
H 3C
C .
:
CH2
:
C
CH2 Br
H 3C
propagation steps
CH3
:
CH2 Br H
H 3C
Br
C
CH2 Br
+
Br.
:
.
C
:
H 3C
H
CH3
:
:
Note that the bromine radical left above can then go back into the reaction and react with another 2-methylpropene molecule. The reaction is ended when all of the chain-carrying radicals (those involved in the propagation steps) combine with other radicals in the termination step (for example, below): . Br
:
Br2
:
:
:
Br.
CH3
:
CH2 Br
CH3
. Br
:
.
C
:
H 3C
H 3C
C
CH2 Br
Br
In contrast to HBr, the acids HCl and HI do not undergo free-radical addition to alkenes, even in the presence of peroxides or O2. Abstraction of H· from HCl is too endothermic, and addition of I· to an alkene is too endothermic. 11.2 Free-Radical Polymerization of Alkenes. The most important free-radical chain reaction conducted in industry is the free-radical polymerization of ethylene to give polyethylene. Industrial processes usually use (t-BuO)2 as the initiator. The t-BuO· radical adds to ethylene to give the beginning of a polymer chain. The propagation part has only one step: the addition of an alkyl radical at the end of a growing polymer to ethylene to give a new alkyl radical at
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the end of a longer polymer. The termination steps are the usual radical–radical combination and disproportionation reactions.
11.3 Free-Radical Substitution of Alkanes. Probably the best-known example of a free-radical reaction is the halogenation of alkanes with Br2 or NBS (N-Bromosuccinimide). This chain reaction is initiated by homolytic cleavage of Br2 induced by light or heat. The propagation step consists of two atom abstraction reactions.
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When more than one kind of H atom is present, the H atom that is removed is usually the one that will leave behind the lowest energy radical. H atoms are never removed from C(sp) or C(sp2), only from C(sp3). Among C(sp3)–H bonds, it is easiest to remove a H if a heteroatom such as N or O is attached to the C. If no such H atom is present, then the H atom that is removed is best allylic or benzylic (the C is attached to a C=C π bond or to a benzene ring). If there are no allylic or benzylic H atoms, then the order of reactivity of H atoms is 3° > 2° > 1°.
Free-radical halogenation is most commonly applied to allylic or benzylic halogenation because the radicals formed at these positions are most stable. Note that alkenes can react with Br2 by either an electrophilic addition reaction, to give 1,2-dibromoalkanes, or by a free-radical substitution reaction, giving 3-bromoalkenes. The former pathway predominates in the dark, whereas the latter pathway predominates in the presence of strong light. Br
Br2
Br2
Br
h! Br
dominates in strong light
dominates in the dark
NBS (N-bromosuccinimide) is often used as the Br source in freeradical brominations. This is to keep the concentration of bromine low and hence reduce competition by electrophilic addition of Br2 to give 1,2-dibromoalkane. In these reactions using NBS in CCl4, it is thought that Br2 is the actual halogenating agent. Br
+
O
N
O
CCl4; heat or
Br
h! NBS
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Br2 is generated as follows: (i) Homolytic cleavage of the N-Br bond of NBS generates a Br atom, which abstracts an allylic hydrogen from the alkene (cyclopentene). Br N
O
. : Br :
+
:
O
.
O
:
heat or h!
N
O
NBS
:
. : Br : (ii)
H
HBr
•
+
The HBr thus formed reacts with NBS to produce a Br2 molecule
H O
Br
:
Br
O
This Br2 molecule reacts with the allylic radical formed earlier and a new bromine atom is produced that can begin the cycle anew . Br + : Br :
•
(iii)
N
Of course, in allylic halogenation, transposition of the double bond can easily occur. For example, in 4,4-dimethylcyclopentene:
Elemental chlorine can be used in free-radical halogenation reactions, too, but these reactions are less easily controlled, because the Cl· radical is more reactive than the Br· radical and hence less selective. The reagents t-BuOCl and SO2Cl2 are used as alternative chlorinating agents. The F· radical is so reactive, and the reaction F–F + C–H → 6
H–F + C–F is so exothermic, that free-radical fluorinations result in violent and uncontrollable exotherms (explosions). At the other extreme, free-radical iodinations of alkanes do not work well at all, as the H· abstraction step is too endothermic.
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